A large variety of molecular sieves have been disclosed in the art as useful in catalysts for hydrocarbon conversion. The most well known are the crystalline aluminosilicate zeolites formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra. The zeolites generally feature pore openings of uniform dimensions, significant ion-exchange capacity and the capability of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure. Zeolites often are characterized by a critical, usually minimum, silica/alumina ratio.
More recently, a class of useful non-zeolitic molecular sieves containing framework tetrahedral units (TO.sub.2) of aluminum (AlO.sub.2), phosphorus (PO.sub.2) and at least one additional element EL (ELO.sub.2) has been disclosed. "Non-zeolitic molecular sieves" include the "ELAPSO" molecular sieves as disclosed in U.S. Pat. No. 4,793,984 (Lok et al.), "SAPO" molecular sieves of U.S. Pat. No. 4,440,871 (Lok et al.) and crystalline metal aluminophosphates--MeAPOs where "Me" is at least one of Mg, Mn, Co and Zn--as disclosed in U.S. Pat. No. 4,567,029 (Wilson et al.). Framework As, Be, B, Cr, Fe, Ga, Ge, Li, Ti or V and binary metal aluminophosphates are disclosed in various species patents. Particularly relevant to the present invention is U.S. Pat. No. 4,758,419 (Lok et al.), which discloses MgAPSO non-zeolitic molecular sieves. Generally the above patents teach a wide range of framework metal concentrations, e.g., the mole fraction of (magnesium+silicon) in Lok et al. '419 may be between 0.02 and 0.98 with a preferably upper limit of 0.35 mole fraction and magnesium concentration of at least 0.01.
The use of catalysts containing a zeolitic molecular sieve and magnesium for isomerization is disclosed in U.S. Pat. Nos. 4,482,773 (Chu et al.) and 4,861,740 (Sachtler et al.), but neither of these references disclose an isomerization catalyst containing non-zeolitic molecular sieves. The use of a catalyst containing a MgAPSO non-zeolitic molecular sieve in hydrocarbon conversion including isomerization is disclosed in the aforementioned U.S. Pat. No. 4,758,419 (Lok et al.). U.S. Pat. No. 4,740,650 (Pellet et al.) teaches xylene isomerization using a catalyst containing at least one non-zeolitic molecular sieve which may be MgAPSO. Neither Pellet et al. nor Lok et al., however, disclose or suggest the narrow criticality of the magnesium content of a non-zeolitic molecular sieve which is a feature of the present invention.
Catalysts for isomerization of C.sub.8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. The former approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs, but the latter approach enhances xylene yield by forming xylenes from ethylbenzene. A catalytic composition and process which enhance conversion according to the latter approach, i.e., achieve ethylbenzene isomerization to xylenes with high conversion, would have significant utility.